A condenser microphone includes an electrostatic type acousto-electric transducer. The acousto-electric transducer includes a diaphragm and a fixed pole that are arranged oppositely via a spacer ring. The fixed pole is spreadingly provided on a support ring (diaphragm ring) under a predetermined tension.
As the diaphragm, a thermoplastic resin film consisting of polyethylene terephthalate or polyphenylene sulfide having a thickness of, for example, 3 to 6 μm, on one surface of which a deposit film is formed by depositing metal, is used. As a general fixed pole, an electrode plate made of a metal such as aluminum is used. In particular, in the case of an electret condenser microphone, an electret dielectric film is integrally provided on the surface (surface opposed to the diaphragm) of the fixed pole.
The electret dielectric film has specific properties (self-polarization properties) of becoming in a polarized state due to corona discharge or the like if a DC high voltage is applied and keeping the polarized state even after the removal of applied voltage.
Generally, it is demanded to install the diaphragm to the support ring under a low tension to position the low limit at a low frequency. However, the fixed pole is arranged on the back surface of the diaphragm, and a polarization voltage due to electret exists between the diaphragm and the fixed pole. Therefore, an electrostatic attraction force is applied to the diaphragm, so that a problem of so-called adsorption of diaphragm to fixed pole occurs.
To solve this problem, Patent Document 1 (Japanese Patent No. 2681207) discloses a technique in which a large number of fine irregularities are formed over the whole region of the diaphragm. The irregularities are provided so that the height from the bottom of a concave part to the top of a convex part is larger than the thickness of the diaphragm.
According to this configuration, under the condition that the low frequency response is the same, the diaphragm provided with irregularities can increase the adsorption stability about 20% as compared with the diaphragm provided with no irregularities. This means that the polarization voltage due to electret can be increased about 20%, thereby increasing the sensitivity by about 2 dB.
However, in the electret condenser microphone, partial variations occur easily in the external electric field due to the electret on the surface of the fixed pole, and the diaphragm is adsorbed in a portion in which the external electric field is high.
Accordingly, a problem with the present invention is to increase the adsorption stability with respect to a fixed pole while improving the low frequency response of a diaphragm especially in an electret condenser microphone.